1. Field of the Invention
The present invention relates to image forming apparatus (which may be analog or digital, e.g. copiers, facsimile machines, printers including an electrostatic printing system, optical printers, etc.) and methods capable of transferring a toner image from an image carrier onto a sheet or like printing medium. More particularly, the present invention relates to toner image transfer devices and methods capable of suppressing transfer toner dust, which tends to be generated when the toner image is transferred from the image carrier onto the printing medium.
2. Discussion of the Background
In conventional analog or digital image forming apparatus, for example, copiers, printers, facsimile machines, etc., a latent image is formed on an image carrier and is developed with dry type toner to obtain a toner image. Such a toner image is usually transferred onto a printing medium such as a sheet of paper or the like under transfer bias voltage. The toner image may be transferred from the image carrier either directly onto the printing medium, or onto an intermediate transfer member for subsequent transfer therefrom to the printing medium.
However, in such conventional analog or digital image forming apparatus, when a toner image formed on an image carrier is transferred onto a printing medium or an intermediate transfer member, some amount of transfer toner dust generally and unavoidably appears on the printing medium or the intermediate transfer member used in a color printer or the like, and accordingly, sharpness of the toner image transferred is generally lost.
One reason why such transfer toner dust appears is that toner particles adjoining each other at an edge portion of the toner image tend to disperse due to the repelling force (coulomb force) existing therebetween, since each of those toner particles is electrically charged with charge of the same polarity before the developing process is executed. Thereby the toner particles spread onto the printing medium or the intermediate transfer member when such toner is transferred onto the printing medium. Accordingly, sharpness of the toner image is lost since the outline of the toner image is disturbed when the toner particles disperse as described above.
Further, it is known that such transfer toner dust more frequently appears when a toner image is transferred onto an intermediate transfer member than when the toner image is transferred onto the printing medium (printing sheet).